1. Field of the Invention
The present invention relates to an automatic transmission for a vehicle. More particularly, the present invention relates to a hydrostatic continuously variable transmission in which a fixed capacity hydraulic pump and a variable capacity hydraulic motor are connected by a hydraulic closed circuit.
2. Description of the Relevant Art
Hydrostatic continuously variable transmissions are well-known. Such transmissions are applied to various vehicles, such as motorcycles. Japanese Patent No. 2527199 describes a control method for a hydrostatic continuously variable transmission, in which an actual rotation speed (Ne) of a crankshaft, or the like, is compared with a target rotational speed (Ne), which has been determined in accordance with predetermined conditions, and an output is controlled by adjusting an inclination angle of a swash plate.
Japanese Patent Laid-Open No. Hei 8-82354 discloses a control method for a continuously variable transmission in which a gear ratio is controlled in a step-by-step manner, as in a manual multi-step transmission (this method is called xe2x80x9cmulti-stage variable transmission controlxe2x80x9d).
When a throttle is closed during downhill driving, for example, the foregoing continuously variable transmission holds the gear ratio at a value immediately prior to downhill driving in order that the vehicle can smoothly decelerate.
The prior art suffers several drawbacks. When a vehicle provided with such a continuously variable transmission and pulling a heavy trailer or the like is driving downhill, it is pushed from behind. Although the throttle is closed in order to maintain the gear ratio, the vehicle is gradually accelerated during the downhill driving. Therefore, there is a need in the prior art for a control method for a continuously variable transmission, which enables a vehicle to reliably drive without acceleration even in the foregoing situation.
It is an object of the present invention to solve one or more of the drawbacks associated with the background art.
A continuously variable transmission of the present invention comprises a control method for controlling the continuously variable transmission, which continuously varies a gear ratio, wherein the gear ratio is changed by a predetermined amount to a LOW side from a normal gear ratio and acceleration is regulated to be below a predetermined value when a throttle is closed and acceleration is above the predetermined value.
In accordance with the present invention, when a throttle is closed and acceleration of a vehicle exceeds the prescribed value, control is performed to shift the gear ratio by a prescribed amount towards the LOW side from that immediately before, so that the acceleration is regulated to be equal to or less than the predetermined value. As a result, a feeling of natural deceleration is obtained when a throttle is closed. Therefore, this is especially effective when driving a vehicle pulling a trailer or the like downhill. Even if the trailer pushes the vehicle from behind, the vehicle can be driven without acceleration.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.